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What were Darwin’s most significant contributions? Ernst Mayr answered: (1) producing massive evidence of evolution, (2) explaining it in terms of natural selection, and (3) thinking about species as populations.

“Population thinking”? Philosophers are still debating what this means. For scientists, however, to think of living things in terms of populations rather than types having each its own essence is a clear and radical departure both from earlier scholarly traditions and from folk biology.

Species evolve, early features may disappear, novel features may appear. From a populationist point of view, a species is a population of organisms that share features not because of a common “nature” but because they are related by descent. A species so understood is a temporally continuous, spatially scattered entity that changes over time.

Population thinking readily extends beyond biology to the study of cultural evolution (as argued by Peter Richerson, Robert Boyd, and Peter Godfrey-Smith). Cultural phenomena can be thought of as populations, the members of which share features because they influence one another even though they do not beget one another the way organisms do and are not exactly copies of one another. Here are three examples.

What is a word, the word “love” for instance? It is standardly described as an elementary unit of language that combines sound and meaning. Yes, but a word so understood is an abstraction without causal powers. Only concrete uses of the word “love” have causes and effects: An utterance of the word has, among its causes, mental processes in a speaker and, among its effects, mental processes in a listener (not to mention hormonal and other bio-chemical processes). This speech event is causally linked, on another time scale, to earlier similar events from which the speaker and listener acquired their ability to produce and interpret “love” the way they do. The word endures and changes in a linguistic community through all these episodes of acquisition and use.

So, the word “love” can be studied as a population of causally related events taking place inside people and in their shared environment, a population of billions and billions of such events, each occurring in a different context, each conveying a meaning appropriate at that instant, and all nevertheless causally related. Scholarly or lay discussions about the word “love” and its meaning are themselves a population of mental and public meta-linguistic events evolving on the margins of the “love” population. All words can similarly be thought of not, or not just, as abstract units of language, but as populations of mental and public events.

What is a dance? Take tango. There are passionate arguments about the true character of tango. From a populationist perspective, tango should be thought of as a population of events of producing tango music, listening and dancing to it, watching others dance, commenting, and discussing the music and the dance, a population that originated in the 1880s in Argentina and spread around the world. The real question is not what is a true tango, but how attributions or denials of authenticity evolve on the margins of this population of acoustic, bodily, mental, and social “tango” events. All culturally identified dances can be thought of as populations in the same way.

What is a law? Take the United States Constitution. It is commonly thought of as a text or, more accurately, since it has been repeatedly amended, as a text with several successive versions. Each version has millions of paper and now electronic copies. Each article and amendment has been interpreted on countless occasions in a variety of ways. Many of these interpretations have been quoted again and again, and reinterpreted, and their reinterpretations reinterpreted in turn. Articles, amendments, and interpretations have been invoked in a variety situations. In other words, there is a population the members of which are all these objects and events in the environment plus all the relevant mental representations and processes in the brain of the people who have produced, interpreted, invoked or otherwise considered versions and bits of the Constitution. All the historical effects of the Constitution have been produced by members of this population of material things and not by the Constitution considered in the abstract. The Constitution, then, can usefully be thought of as a population, and so can all laws.

Population thinking is itself a population of mental and public things. Philosophers’ discussions of what population thinking really is are members of this population. So is the text you just read, and so is your reading of it.